Memoization implemented in JavaScript

memoization.js

function memoize(fun) {
  var mem = {};
  return function() {
    var args = arguments.toString();
    if(mem[args] == undefined) {
      mem[args] = fun.apply(this, arguments);
    }
    return mem[args];
  }
}

// usage:

function slow(x) {
  for(var p = 0; p < 999000000; p++) { }
  return x;
}

var memoSlow = memoize(slow);

slow(10); // runs very slow

memoSlow(10); // runs very slow

memoSlow(10); // runs very fast